1. Field of the Invention
The present invention relates to a vibrometer and interferometer device for metrology measurement and, more particularly, to a laser Doppler interferometer device to be called "AVID" which stands for Advanced Vibrometer/Interferometer Device herein.
2. Description of the Prior Art
Laser Doppler vibrometer and interferometer devices have been used in various metrology areas. Their applications in the direct access storage device (DASD) area have certainly been practiced for a long time. However, an optical system that can be easily miniaturized so that accurate measurements can be obtained while the optical system is traversed with the measurement objects has not been readily available. The need to develop such a system is significant for optical glide technology since slider vibrations and slider/disk spacings need to be measured as a slider transverses across a disk surface.
FIG. 1 shows a conventional optical arrangement on an optical unit applying a basic principle of a Michelson interferometer device. A collimated light from a visible diode laser 10 passes through an aperture mirror 16 and then is split into two interfered arms by a polarization beamsplitter PBS1 20. A first light beam 1 and a second light beam 2 each possesses an orthogonal polarized state respectively. In a conventional example shown in FIG. 1, two outgoing light beams are linearly polarized. The first light beam 1 passes through two 45-degree reflective mirrors 22, 24 arranged side by side and a polarization beamsplitter PBS2 30, and then it is focused to a point on a test target by a doublet focusing lens L1 26. The second light beam 2 passes through two 45-degree reflective mirrors 32, 34 arranged side by side and the polarization beamsplitter PBS2 30, and then it is focused to a point on the test target by another doublet focusing lens L2 36. The two outgoing light beams and two returning object beams are off-axially incident to different positions on the two focusing lenses L1 26 and L2 36, The two returning object beams remain linearly polarized and are orthogonal with respect to each other after they are recombined at PBS1 20. One right circularly polarized light beam and one left circularly polarized light beam are generated after the two returning object beams traverse a quarter waveplate QW 45 oriented at 45 degrees. The sum vector of the two circularly polarized light beams is still a linearly polarized light beam but with an inclined angle relative to a horizontal axis due to the movement of an object. Then the resultant linearly polarized light beam is divided into two interfered light beams by a non-polarization beamsplitter NPBS 50. Two polarizers P1 52 and P2 54, whose axes are 45 degrees apart, are utilized to make the 90-degree phase differences of the light intensities measured at respective photodiodes PD1 62 and PD2 64 ideal. Velocity and position signals can be easily decoded by using the signal processing schemes. The two returning object beams E1 and E2 can be expressed as ##EQU1## where f.sub.1 is the light wave frequency; and f.sub.d1 and f.sub.d2 are the Doppler shifted frequencies created by the motion of the two object surfaces 1 and 2, respectively; and .phi. is the relative phase difference between the two object beams E1, E2 generated from the optical path differences and phase angle variations due to reflections, etc. After the two object beams E1, E2 pass through the quarter waveplate QW 45 the combined light vector can be expressed as follows: ##EQU2## which is the coherent sum of the two circularly polarized light beams, one is right circularly polarized and one is left circularly polarized, and can be viewed as a linearly polarized light beam with its polarization axis located at 2.pi.(f.sub.d1 -f.sub.d2)t+.phi.!/2. In order to perform a quadrature measurement to remove directional ambiguity, a method similar to the Pocket Servowriter Project approaches can be adopted. That is, the two polarizers P1 52, P2 54 whose axes are 45 degrees apart can be used as shown in FIG. 1 to make the light intensity measured at respective photodiodes PD1 62, PD2 64 be expressed as EQU I.sub.1 .varies.1+sin(2.pi.(f.sub.d1 -f.sub.d2)t+.phi.)
and EQU I.sub.2 .varies.1+cos(2.pi.(f.sub.d1 -f.sub.d2)t+.phi.)
If a standard quadrature signal detection technique is performed to the above two signals, a relative position between the two object surfaces can be obtained in real time. In DASD applications, this setup gives the clearance measurement, which can be used to set the clip level in glide. A versatile differential laser interferometer is then created. Furthermore, if the electric signals from the photodiodes PD1 62 and PD2 64 are mixed with a cosine signal and a sine signal which are electronically generated with frequency f.sub.c separately, the following can be obtained. EQU cosine channel: cos2.pi.(f.sub.d1 -f.sub.2)t+.phi.!.multidot.cos(2.pi.f.sub.c t) EQU =1/2{cos2.pi.(f.sub.d +f.sub.c)t+.phi.!+cos2.pi.(f.sub.d -f.sub.c)t+.phi.!},
and EQU sine channel: -sin2.pi.(f.sub.d1 -f.sub.d2)t+.phi.!.multidot.sin(2.pi.f.sub.c t) EQU =1/2{cos2.pi.(f.sub.d +f.sub.c)t+.phi.!-cos2.pi.(f.sub.d -f.sub.c)t+.phi.!},
where f.sub.d =f.sub.d1 -f.sub.d2 is the relative Doppler phase shift between the two object surfaces. Summing these two signals yields cos{2.pi.(f.sub.d1 -f.sub.d2)t+.phi.!. Sending this signal into a simple frequency-to-voltage converter will yield a velocity signal due to the Doppler effect. This is the fundamental configuration of the conventional laser Doppler vibrometer and interferometer. Thus, both velocity and displacement measurements can be performed thereon.
To satisfy the stringent demands of today's ultra-high performance machinery such as optical/magnetic disk drivers, digital video disk drivers, etc., an interferometer device with high accuracy and wide bandwidth is becoming an essential metrology tool. Comparing the present invention with the conventional optical metrology instruments, several main features are needed for interferometer devices to be adopted into these ultra-high precision high performance machinery measurements. They are: (1) no surface modification to measurement samples, (2) absolute/differential detection, (3) ability to accommodate samples with significantly different reflectivity, (4) nanometer resolutions and (5) a megahertz bandwidth.